St. John is fabulous. My favorite island of all. I don't consider StJ easy to get to from IAH.
Search for shore diving and you'll find shore snorkeling. My caveat below is no night life except underwater!
Roatan, Honduras - easy direct flight from IAH - has Cocoview which truly caters to divers but there a number of return snorkelers there as well. Huge front yard for snorkeling. Amazing front yard.
Curacao - Go West apartments has amazing snorkeling in front of apartments. They have a deal that includes car rental with apartment. Lots of places for easy snorkel. However steep steps to water usually. Amazing island with amazing people. Best stewed goat.
Wakatobi, Indonesia - high end - best of all worlds. Good shore dives with boats as well. High end price. Amazing service. Amazing protected reef. The most abundant life I've ever seen.
And if you want to learn to dive in Houston, I can give you some excellent recommendations.